1. The Field of the Invention
The present invention relates to data storage associated with computers and data processing systems. Specifically, the present invention relates to methods used to recover from a computer failure in a system having a plurality of computer systems, each with its own mass storage device.
2. Background and Related Art
Computer networks have greatly enhanced mankind's ability to process and exchange data. Unfortunately, on occasion, computers partially or completely lose the ability to function properly in what is termed a “crash” or “failure”. Computer failures may have numerous causes such as power loss, computer component damage, computer component disconnect, software failure, or interrupt conflict. Such computer failures can be quite costly as computers have become an integral part of most business operations. In some instances, computers have become such an integral part of business that when the computers crash, business operation cannot be conducted.
Almost all larger businesses rely on computer networks to store, manipulate, and display information that is constantly subject to change. The success or failure of an important transaction may turn on the availability of information which is both accurate and current. In certain cases, the credibility of the service provider, or its very existence, depends on the reliability of the information maintained on a computer network. Accordingly, businesses worldwide recognize the commercial value of their data and are seeking reliable, cost-effective ways to protect the information stored on their computer networks. In the United States, federal banking regulations also require that banks take steps to protect critical data.
One system for protecting this critical data is a data mirroring system. Specifically, the mass memory of a secondary backup computer system is made to mirror the mass memory of the primary computer system. Write requests executed in the primary mass memory device are transmitted also to the backup computer system for execution in the backup mass memory device. Thus, under ideal circumstances, if the primary computer system crashes, the backup computer system may begin operation and be connected to the user through the network. Thus, the user has access to the same files through the backup computer system on the backup mass memory device as the user had through the primary computer system.
However, the primary computer system might crash after a write request is executed on the primary mass memory device, but before the request is fully transmitted to the backup computer system. In this case, a write request has been executed on the primary mass memory device without being executed on the backup mass memory device. Thus, synchronization between the primary and backup mass memory devices is lost. In other words, the primary and backup mass memory devices are not perfectly mirrored, but are slightly different at the time of the crash.
To illustrate the impact of this loss in synchronization, assume that the primary and backup mass memory devices store identical bank account balances. Subsequently, a customer deposits money into an account and then shortly thereafter changes his mind and withdraws the money back from the account. The primary computer system crashes just after the account balance in the primary mass memory device is altered to reflect the deposit, but before the write request reflecting the deposit is transferred to the backup computer system. Thus, the account balance in the backup mass memory device does not reflect the deposit. When the customer changes his mind and withdraws the money back out from the account, the account balance in the backup memory device is altered to reflect the withdrawal. When the primary computer system is brought back into operation, the account balance from the backup mass memory device is written over the account balance in the primary mass memory device. Thus, the account balance reflects the withdrawal, but does not reflect the deposit.
Another disadvantage of this system is that when that primary computer system is brought back into operation, the entire backup mass storage device is copied back to the primary mass storage device in what is termed a “remirror”. The copying of such large amounts of data can occupy a significant time and be disruptive to transactional operations.
Therefore, a backup computer system and method are desired that do not result in the above-described loss of synchronization, and that do not require a complete remirror.